The invention relates to an apparatus for wire-free transmission of the least one measured value from moving parts, as claimed in the preambles of the independent claims 1, 5 and 9. An apparatus of the this generic type and as claimed in claims 1 and 5 has been disclosed in EP 0 201 149 A2. An apparatus of this generic type and as claimed in claim 9 has been disclosed in DE 31 07 947 A1.
The invention is based on the problem of determining the pressure and other measured values from the rotating tire, converting them into an electrical signal and indicating them or, for example, evaluating them for pressure-loss warning messages. The determination of the pressure itself and its conversion into all electronic variable presents no problems whatsoever based on the prior art, and suitable mechanically or capacitively based sensors as well as integrated solutions based on semiconductors have been known for a long time.
The actual problem in the case of the vehicle is represented by the data transmission and the power supply of the pressure sensor since the high and severely fluctuating rotation speed of the tire together with the steering and chassis movements and the severe dirt contamination prevents not only transmission via wires, for example via sliding contacts, but also optical or acoustic transmission.
Systems which operate on an inductive or radio basis are therefore used in high-quality vehicles. To this end, a generator produces a supply frequency which is made available in the vicinity of the tire by means of a coil or antenna. This supply frequency can then be absorbed or amplified by a tuned circuit or other resonator, with the resonator being influenced by a pressure sensor. This influencing of the fundamental frequency can then be measured and evaluated. This is disclosed, for example, in EP-A-0 450 653, which is similar to the method of inductive train protection.
However, in all these methods, the high susceptibility to defects is disadvantageous, resulting in particular from the use of the same frequency for supplying the measured value transmitter and for measurement. In consequence, such attenuations or amplifications of the measurement frequency could also be misinterpreted as the measured value, although they are not caused by the measure value transmitter.
It would thus be obvious to accommodate an active circuit in the measured value transmitter, which is supplied by a supply frequency and responds at a second frequency. Unfortunately, owing to the rapid rotation of the wheel, the supply is, however, ensured over an acceptable time period only if the distance between the measured value transmitter and the supply coil or antenna fluctuates only to a minor extent. This makes it necessary to position the measured value transmitter in the vicinity of the axle, and thus makes it necessary to use a special rim which, for its part, results in high costs.
The invention is thus based on the object of transmitting the measured value reliably even if the transmission conditions are fluctuating severely, and without a dedicated power supply for the measured value transmitter.
The object is in each case achieved according to the invention by the apparatuses which are specified in the independent patent claims and its functions will be explained in the following text. Advantageous embodiments are described in the dependent claims.
The system according to the first alternative of the invention comprises a measured value transmitter which is located in the tire and does not have its own power supply, as well as a measurement system which is mounted in a fixed position in the vehicle and comprises one or more antennas and one or more electronic assemblies.
The supply frequency is produced by one or more generators and is emitted via an antenna in the vicinity of the tire. This supply frequency is now received by the measured value transmitter, for example via a tuned circuit, and is passed on to a non-linear element, for example a varactor diode. Harmonics are now formed at this element or, if there are a plurality of supply frequencies, their sum and difference frequencies. The measured value transmitter also contains a resonator which is tuned to a suitable harmonic, or sum or difference frequency, and which can be influenced by the measured value. It is now alternatively possible to measure the tire pressure either by varying the resonant frequency by means of a capacitive transmitter or by switching the resonator on or off by means of switches.
The measurement frequency which is thus produced in the measured value transmitter and is emitted can now easily be received using a suitable receiver, without any interference from the supply frequency, and can be evaluated to determine the measured value. The measured value is then made available by the receiver for further evaluation and indication.
The receiver is now no longer influenced by the supply frequency, and the dynamic requirements for its input stage are reduced. This also allows higher measurement and supply frequencies to be used, as a result of which, even at high wheel rotation speeds, a sufficient number of cycles are transmitted for reliable identification or frequency determination of the measurement frequency every time the measured value transmitter moves past the fixed antennas. Furthermore, the transmission range is now increased at the higher frequencies that are now possible.